1. Field of the Invention
This invention relates to a semiconductor valve for high voltage applications, comprising a plurality of electrically series-connected semiconductor elements and a plurality of cooling bodies adapted to be traversed by a liquid coolant, the semiconductor elements and the cooling bodies being arranged in a stack in such a way that each semiconductor element is arranged between two cooling bodies. The semiconductor valve also includes a pressure means arranged to generate a compressive force acting on the cooling bodies and semiconductor elements in the longitudinal direction of the stack. The semiconductor valve also includes voltage divider sections which are connected in parallel with each semiconductor element, each voltage divider section comprising at least one resistor.
2. Description of the Prior Art
Semiconductor valves of the above-mentioned kind are previously known from Mellgren et al. U.S. Pat. No. 3,536,133. Such valves are used, for example, in convertors for HVDC power transmission systems, or as parts of the switching member used for static reactive power compensation.
In semiconductor valves of the above-mentioned kind, the voltage divider sections form a chain arranged in parallel with the individual semiconductor elements in such a way that each element is parallel-connected to a voltage divider section. Each such voltage divider section normally includes one or more resistors. Valves with such voltage divider sections are previously known, for example, from U.S. Pat. Nos. 3,794,908 and 4,360,864.
The resistors or the voltage divider sections have a high power dissipation, typically of the order of magnitude of several hundred watts. IF the resistors are air-cooled, this has several serious disadvantages. The resistors have to be provided with cooling flanges, or the like large surface area structures, so as to ensure adequate heat emission to the surrounding air and prevent too great a temperature rise. Further, it is necessary to ensure that there is a sufficient flow of cooling air past the resistors in order to carry away the waste heat. For these reasons, a semiconductor valve of the above-described kind, if provided with air-cooled resistors, tends to become relatively bulky. An additional disadvantage is that the emission of power from the resistors will be high, which results in a high air temperature in the valve and in the surrounding environment and, therefore, an increased ambient temperature for other components and equipment disposed adjacent to the semiconductor valve, if special additional cooling measures are not taken.
It is previously known, for example from U.S. Pat. No. 2,274,381, to liquid-cool high-power resistors. However, making each of the many resistors in the voltage divider sections of a semiconductor valve of the kind described liquid-cooled, would result in a complicated valve having many connections and conduits for the liquid coolant. With these coolant conduits, the semiconductor valve would still be relatively bulky and there would still be some heat emission from the resistors and the coolant conduits that would raise the ambient temperature in the vicinity of the valve.
From U.S. Pat. No. 4,178,630 it is previously known to mount the resistors forming the voltage divider section on a cooling bar for the semiconductor elements of the valve, which cooling bar is provided with means permitting it to be cooled with a liquid coolant. A certain proportion of the waste heat from the resistors will then be extracted by the liquid coolant which traverses the cooling bar, but the heat emission to the surrounding air will still be relatively high.